MDL-39814 frontend: improvements to course/block editing
MDL-39814 frontend: improved display of activities and the menu
MDL-39814 frontend: introduced an action_menu_action class
MDL-39814 frontend: increased size of editing icons for activities
MDL-39814 frontend: redeveloped overlay to correct tab-flow

Also:
 * fixed zindex issue + spinner placement
 * added notes to course format and theme upgrade texts
 * fixed behat tests after action menu changes
MDL-39814 actionmenu: improved alignment, constraint handling, and accessibility
MDL-39814 frontend: added options to disable new action menus

Also:
 * Tidied up patch as per peer-reviews